Diffractive optical elements may be implements in a wide variety of different forms, such as Fresnel zone plates, diffusers, kinoforms, phase gratings, and holograms, and may be used in a wide variety of different optical applications, including high resolution imaging systems and fiber optic coupler interconnect systems. Recently, diffractive optical elements have been developed to perform complicated phase transformations of incident radiation, such as wavefront conversion. Diffractive optical elements may be reflection-type diffractive optical elements or they may be transmission-type diffractive optical elements.
In general, diffractive optical elements should have high diffraction efficiencies. In order to achieve 100% diffraction efficiency, a continuous phase profile is required within any given period, but device structures that provide such continuous phase profiles are difficult to manufacture. A continuous phase profile may be approximated, however, by a multilevel diffractive optical element having a set of discrete phase levels. The larger the number of discrete phases levels, the better the approximation to the corresponding continuous phase profile. Such diffractive optical elements may be constructed with relatively high efficiencies and are easier to manufacture than diffractive optical elements providing continuous phase profiles. A multilevel diffractive optical element typically is manufactured by generating a set of binary amplitude etch masks and serially masking and etching multiple levels of a material structure. The step heights of the levels of a multilevel diffractive optical element may be the same or they may be different (e.g., the step heights may be weighted binarially).
The multilevel surface profiles of stepped diffractive optical elements may be manufactured using standard, semiconductor integrated circuit fabrication techniques. However, etch processes should be optimized to achieve accurate and repeatable etch depths for the different phase levels. In general, etch processes that rely on control of etch rate and etch time are difficult to implement and often suffer from microloading proximity effects in which the local etch rate varies with the local pattern density such that the resulting phase shift of each level varies with pattern density. U.S. Pat. No. 6,392,792 has proposed a method of fabricating a reflection-type multilevel diffractive optical element from an etch stack consisting of alternating layers of two different materials that exhibit good etch selectivity properties such that adjacent stack layers provide natural etch stops for each other. The '792 patent describes an etch stack with alternating layers of silicon and silicon dioxide that have substantially equal heights. After the etch stack layer has been etched, an overcoat of a multilayer reflection stack is formed over the etched structure to complete the device.